1. Field of Invention
The present invention relates to vessels, and more particularly to methods, apparatus, computer-readable media and signals for evaluating a vessel.
2. Description of Related Art
Many applications benefit from the evaluation of a vessel. For example, where the vessel includes a coronary artery or other blood-carrying vessel, coronary artery diseases or other vessel disease can result in the narrowing or alteration of the shape of a vessel by various disease processes. Such narrowing or alteration in the shape of a vessel can be diagnostic of a disease process and dictate the approach or approaches that are desirable to follow in the treatment of a patient or other subject. Blockages or narrowing (stenosis) of vessels can lead to reduced blood flow to tissues and subsequently reduced oxygen delivery to those tissues, potentially causing many serious medical problems, such as heart attacks or strokes, for example. Blockages and narrowing can occur for many reasons.
Atherosclerosis, or hardening of the arteries, may cause a number of diseases. These diseases stem from the loss of normal function of the blood vessels caused by the presence of plaques that may gradually encroach on the lumen of the artery. As a result, atherosclerotic vessels may be unable to provide adequate flow of blood to a particular downstream organ, resulting in ischemia. In atherosclerosis, vessels develop plaques or atheromas within the intima of the diseased arteries. A variety of cell types accumulate in the developing atherosclerotic plaque, including modified smooth muscle cells, monocytes/macrophages, and T lymphocytes. The presence of leukocytes in human atherosclerotic plaques can lead to subacute or chronic inflammation. Monocytes and T lymphocytes from the bloodstream invade the damaged arterial wall, which can lead to an accumulation and the formation of an early lesion. An advanced atherosclerotic lesion generally consists of a cholesterol- and lipid-rich core that contains lipid-laden macrophages and is covered by a fibrous cap of connective tissue. There have also been associations between common bacterial pathogens, like chlamydiae, and atherosclerosis.
In most diagnostic testing, the presence or absence of an abnormality and the severity of the abnormality can be indicative of a disease process. However, the effectiveness of diagnostic tests may be limited by the sensitivity of the testing method in detecting an abnormality, and may also be limited by the ability to interpret the results. In conventional coronary angiography, for example, where the vessel is a coronary artery, the traditional diagnostic parameter is the Percent Diameter Stenosis (% DS) value, which effectively measures the diameter of the artery at a site of a lesion or other obstruction, relative to a reference diameter measurement at an “unobstructed” site within the same artery, adjacent the obstruction. A % DS value that is low or close to zero indicates virtually no obstruction or stenosis at the lesion site, whereas at the other extreme, a % DS value close to 100% would indicate almost total blockage at the lesion site. However, the reliability of the conventional % DS value as an indicator of actual stenosis is dependent upon the ability of the diagnostician to correctly identify a “healthy” nearby location that is unobstructed, at which the reference diameter measurement can be obtained.
In practice, the diagnostician uses the angiographic images to visually distinguish between the lesion or diseased site and the healthy, unobstructed site, selecting the location of a visible lesion or focal stenosis for the lesion site measurement, and selecting a nearby location having no such visible lesion or focal stenosis for the reference diameter measurement. However, examining the results of invasive methods such as intravascular ultrasound, the present inventors have found that many areas that appear to be normal in the angiographic images and therefore appear to be suitable for the reference diameter measurements, are in fact affected by atheroma. The atheroma accumulation induces outward expansion or “centrifugal remodeling” of the artery, thereby preserving the lumen of the vessel and rendering the angiographic appearance of the lumen as “normal”, when in fact it is diseased. This camouflaging effect is particularly prevalent at the early stages of atheroma. This misleads the diagnostician into improperly selecting a diseased artery site as a measurement site for the supposedly normal, unobstructed reference diameter, which adversely affects the diagnostic value of the angiogram with respect to detection of atherosclerosis, especially in its early stages. Typically, such an error results in the % DS value being lower than it would have been if a truly healthy site had been used for the reference value, thereby resulting in a likelihood that the % DS value will fail to reveal an underlying stenosis caused by early-stage atherosclerosis.
Although intravascular ultrasound can detect some such diseased sites that appear visually normal in angiographic images, intravascular ultrasound is an invasive method, and is typically applied only as an adjunct to angiography. Intravascular ultrasound is typically not suitable for the general population of patients undergoing angiography, especially those that do not have any other need for the insertion of large hardware (such as that required to perform percutaneous coronary intervention such as balloon/stent angioplasty) into an apparently normal-looking artery.
Other methods, such as carotid ultrasound, computed axial tomography, or magnetic resonance imaging, may assist in screening for detection of early-stage atheroma. However, these relatively new techniques are not expected to displace current measurement techniques such as angiography. These techniques are also expensive, and typically do not diminish the number of patients undergoing angiography. Indeed, wider spread use of these other methods may increase the need to proceed to angiography.
Accordingly, there is a need for an improved way of evaluating a vessel.